WDM (Wavelength Division Multiplex) transmission using a plurality of channels is used in a high speed optical communication system in order to enlarge transmission capacity, as operation speed of each component gets closer to a limit. Application of the WDM transmission to a transmission having a relatively short distance of a few tens km or less is under review, although the WDM transmission is applied only to a transmission having a distance of a few hundreds km or more. In the system, an optical amplifier is not provided on a transmission path, for cost reduction. For example, a transmitter modulates a plurality of signals having a different wavelength, multiplexes the modulated signals and transmits the multiplexed signal without amplifying. A receiver amplifies the multiplexed signal at a semiconductor optical amplifier in block in order to establish the distance extension, branches the amplified signal and receives the branched signals at light-receiving elements.
It is preferable that the light-receiving element of the receiver has a large dynamic range in order to use an optical fiber having variation of distance. For example, it is preferable that the light-receiving element has the dynamic range of 20 dB or more with respect to distance variation of 0 km to a few tens km of an optical fiber. There is a case where optical intensity is different from each other between the channels.
Japanese Patent Application Publication No. 2008-166719 (hereinafter referred to as Document 1) discloses an art in which input power and output power are monitored with respect to a specific wavelength, and a gain is controlled according to a table storing characteristics of a semiconductor optical amplifier. Japanese Patent Application Publication No. 2000-252923 (hereinafter referred to as Document 2) discloses an art in which wavelength dependency of the optical intensity (optical power) is corrected with a gain equalizer or the like.
However, the art of Document 1 fails to secure a large dynamic input range with respect to a plurality of channels because the art of Document 1 monitors a single wavelength. With the art of Document 2, cost for components may be increased.